A conventional method for manufacturing frozen aerated products, in particular ice cream, comprises operations for mixing, homogenizing, pasteurizing, freezing and hardening the mixture to be frozen. Aeration of the mixture or expansion is carried out at the freezing stage in a proportion such that the volume increases by 70 to 120%. On leaving the freezing device or freezer, the temperature of the aerated mass is typically -5 to -6.degree. C. This is then hardened at -40 to -45.degree. C. in a hardening chamber, until the core temperature of the product reaches -18.degree. C. or less for bulk products or -30.degree. C. for extruded products as bars.
Attempts have been made to lower the temperature of the mass on leaving the freezer, for reasons of energy saving and with the aim of improving its texture, for example in the sense of greater smoothness. However, with conventional equipment, insurmountable problems of high viscosity of the mass of ice cream at temperatures below -7 to -8.degree. C. are encountered. These problems have been in part resolved by using two scraped surface freezers in series, the first, which is conventional, delivering aerated ice cream at around -7.degree. C. and the second, being specially designed to process the highly viscous mass so as to lower its temperature to around -10.degree. C.
Similarly, EP-A-0561118 describes a three-stage method for producing ice cream at low temperature, down to around -20.degree. C. This is an output temperature at which the hardening stage can be completely eliminated for bulk products and appreciably shortened for extruded products. In the first, so-called pre-expansion, stage, air is incorporated into the mixture to be frozen at a positive temperature. In the second, the aerated mass is cooled in a scraped surface exchanger, emerging at around -6.degree. C. During the third, a screw device cools the mass to around -20.degree. C.
U.S. Pat. No. 5,024,066 concerns a two-stage system. In the first, a pre-expansion takes place during which air is added to the mass to be frozen at a positive temperature. In the second, the aerated mass is cooled, by means of an Archimedes screw with a roughened surface fitted with scraper blades on its periphery, to a sufficiently low negative temperature to ensure that the frozen mass has a stable texture, allowing the products to be stored directly in cold store.